While you can delete Snapchat messages easily, you have to understand that it is not sufficient to completely erase the messages. The deleted messages can be retrieved, so you have to erase all traces of Snapchat messages from your iPhone. This can be done in many ways.

iMyFone Umate Pro (http://www.czqsoft.com/detail/iPhone-iPad-iPod-Data-Eraser) (and also the Mac version of iMyFone Umate Pro) is a great tool for deleting data from your iPhone. The tool helps in deleting all your junk files, and the entire process is a free one. You can do a permanent erasure of all logs and caches in your iPhone's Snapchat app.

This method ensures your privacy is not compromised and further, it also assists in freeing up much-needed space for your device enabling to boost the performance of your device.

Clear all Snapchat files that are corrupted (http://www.czqsoft.com/useguides/How-to-Delete-Snapchat-Messages-Easily-by-the-Snap-History-Eraser)

While Snapchat is an efficient interactive platform, it has its own drawbacks like the build up of temporary files that eat up vital space in your device leading to the device becoming too slow for your liking. iMyFone Umate Pro is handy in such a situation, as it helps to clear off all the temporary files freeing up sufficient storage space in the device.

To deleted snapchat messages completely on iPhone, maybe you need to try iOS Data Eraser program (https://www.androidtoiphonetransfer.com/ios-data-eraser.html). It helps us delete personal files containing your private information from iPhone, such as contacts, messages or personal videos/photos and so on. And the deleted data can't be recovered by any tool.